A Flexible Printed Circuit (FPC), as a special base material that connects electronic components, has excellent performances such as being light, thin, diverse in structure, and bend-resistant. It can be widely used in folding mobile phones, liquid crystal displays, notebook computers, tape-carrier IC packaging substrate and other high-end areas.
In recent years, with the rapid development of the electronics industry, electronic products further develop towards miniaturization, lightweight, and high-density assembly. Driven by the high-frequency trend of communication systems, the third generation of mobile phones has entered the market in a large amount. In addition, the integration of mobile phones will also promote mobile phone components to have a high frequency and a high speed rapidly. The high-frequency trend of mobile phone key components is bound to be unavoidable. The electromagnetic interference problem inside and outside components resulted from being driven by the high-frequency and high-speed trend will gradually become serious. Therefore, an important indicator of a functional flexible circuit board is electromagnetic shielding (EMI shielding). A shielding film has become essential auxiliary material of a circuit board.
In the circuit board industry, an electromagnetic shielding film is affixed after a covering film on the circuit board opens a window, the noise generated by the signal line is conducted into the circuit board stratum through the shielding film, and the electromagnetic interference inside and outside shielding components is achieved. This using method requires the design of a grounded PAD on the circuit board, a window must be opened in the covering film, and the amount of adhesive overflow of the covering film should be strictly controlled. The complexity of the circuit board processing technology will be increased, while the size of the grounded PAD will affect the shielding effect. The biggest drawback is that the grounded PAD must be designed, so the space of the circuit board cannot be fully utilized.
The Chinese Patent CN102026529 A, entitled as Shielding Film, Shielding Circuit Board Having The Same, And Method Of Grounding Shielding Film, discloses a conductive member including a metal layer connected to an external grounding member in the state of contact and a conductive grounding layer including conductive particles, wherein the grounding is formed in such manner that the conductive particle pierces the insulating covering film layer of the shielding film connected thereto and is finally connected to the stratum. Although this method can achieve free grounding of the shielding film and overcome the above technical difficulties, the size of the conductive particle is relatively high, and the cost is higher. Particles with a special shape and a larger diameter are needed to pierce the insulating covering film layer of the shielding film, while the particle size distribution of conductive particles should be relatively uniform. If there are conductive particles whose particle size is lower than the average particle size, there will be insulating covering film layer of the shielding film that cannot be pierced locally, thus affecting the grounding.